Electronic Toll Collection (ETC) systems enable drivers to pay road tolls without stopping their vehicles. An ETC system is especially useful for highways, bridges and tunnels with heavy traffic.
A Road Side Unit (RSU) broadcasts wireless signals at a certain frequency, such as 14 KHz square waves modulated by 5.83 GHz or 5.84 GHz carrier waves according to a standard (e.g., national standard of China).
An On Board Unit (OBU) uses a wake-up circuit, which will wake up the primary circuit in the OBU when one or more preset conditions are met, for example when the OBU detects the 14 KHz square wave transmitted by the RSU.
The On Board Unit (OBU) comprises a Resistor-Capacitor (RC) oscillator. The RC oscillator is sensitive to environment conditions, and the RC oscillator will shift. Therefore the RC oscillator may need to be calibrated from time to time.